The horns of most animals have a bony core covered by a thin sheath of keratin, the same substance as hair and nails. Rhino horns are unique, however, because they are composed entirely of keratin. Scientists had been puzzled by the difference, but the new study has revealed an interesting clue: dark patches running through the center of the horns.

Researchers conducted CT scans on the horns and found dense mineral deposits made of calcium and melanin in the middle.

The calcium deposits make the horn core harder and stronger, and the melanin protects the core from breakdown by the sun’s UV rays, the scientists report. The softer outer portion of the horn weakens with sun exposure and is worn into its distinctive shape through horn clashing and by being rubbed on the ground and vegetation.

The study also ends speculation that the horn was simply a clump of modified hair.

“The horns most closely resemble the structure of horses’ hoofs, turtle beaks and cockatoo bills” said Tobin Hieronymus, lead author on the study.